Information processing on a head-mountable device

ABSTRACT

Example methods and systems determine a position of a portion of a human eye based on electromagnetic radiation reflected from the surface of the human eye. A sensor associated with a computing device can be calibrated in response to an event. The computing device can receive data indicative of electromagnetic radiation reflected from a human eye. The computing device can determine a position of a portion of the human eye based on the received data indicative of electromagnetic radiation. The computing device can generate an indication including the position of the portion of the human eye. The computing device can transmit the indication from the computing device. In some embodiments, the data indicative of electromagnetic information can be provided by electromagnetic emitter/sensors mounted on a wearable computing device directed toward a human eye of a wearer of the wearable computing device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Pat. App. No. 61/584,152, filedJan. 6, 2012, entitled “Methods for Eye-Tracking on a Head MountableDisplay”, now pending, the contents of which are incorporated byreference herein for all purposes.

This application is related to U.S. patent application Ser. No.13/235,201, filed Sep. 16, 2011, entitled “Wearable Computing Systemwith Eye-Movement Unlock”, now pending.

BACKGROUND

Unless otherwise indicated herein, the materials described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Computing devices such as personal computers, laptop computers, tabletcomputers, cellular phones, and countless types of Internet-capabledevices are increasingly prevalent in numerous aspects of modern life.Over time, the manner in which these devices are providing informationto users is becoming more intelligent, more efficient, more intuitive,and/or less obtrusive.

The trend toward miniaturization of computing hardware, peripherals, aswell as of sensors, detectors, and image and audio processors, amongother technologies, has helped open up a field sometimes referred to as“wearable computing.” In the area of image and visual processing andproduction, in particular, it has become possible to consider wearabledisplays that place a very small image display element close enough to awearer's (or user's) eye(s) such that the displayed image fills ornearly fills the field of view, and appears as a normal sized image,such as might be displayed on a traditional image display device. Therelevant technology may be referred to as “near-eye displays.”

Near-eye displays are fundamental components of wearable displays, alsosometimes called “head-mounted displays” (HMDs). A head-mounted displayplaces a graphic display or displays close to one or both eyes of awearer. To generate the images on a display, a computer processingsystem may be used. Such displays may occupy a wearer's entire field ofview, or only occupy part of wearer's field of view. Further,head-mounted displays may be as small as a pair of glasses or as largeas a helmet.

Emerging and anticipated uses of wearable displays include applicationsin which users interact in real time with an augmented or virtualreality. Such applications can be mission-critical or safety-critical,such as in a public safety or aviation setting. The applications canalso be recreational, such as interactive gaming.

SUMMARY

In one aspect, an example computer-implemented method can include: (a)in response to an event, calibrating a sensor associated with acomputing device, (b) receiving data indicative of electromagneticradiation reflected from a human eye at the sensor associated with thecomputing device, (c) determining a position of a portion of the humaneye based on the received data indicative of electromagnetic radiationusing the computing device, (d) generating an indication including theposition of the portion of the human eye using the computing device, and(e) transmitting the indication from the computing device.

In another aspect, an example computing device can include a processor,one or more electromagnetic-radiation emitter/sensors, a non-transitorycomputer-readable medium and program instructions stored on thenon-transitory computer-readable medium. The program instructions can beexecutable by the processor to cause the wearable computing device toperform functions. The functions can include: (a) in response to anevent, calibrating at least one electromagnetic-radiation emitter/sensorof the one or more electromagnetic-radiation emitter/sensors, (b)receiving data indicative of electromagnetic radiation reflected from ahuman eye at the at least one electromagnetic-radiation emitter/sensor,(c) determining a position of a portion of the human eye based on thereceived data indicative of electromagnetic radiation, (d) generating anindication including the position of the portion of the human eye, and(e) transmitting the indication.

In yet another aspect, an article of manufacture can a non-transitorycomputer-readable medium having instructions stored thereon that, if theinstructions are executed by a computing device, can cause the computingdevice to perform functions. The functions can include: (a) in responseto an event, calibrating a sensor, (b) receiving data indicative ofelectromagnetic radiation reflected from a human eye from the sensor,(c) determining a position of a portion of the human eye based on thereceived data indicative of electromagnetic radiation, (d) generating anindication including the position of the portion of the human eye, and(e) transmitting the indication.

These as well as other aspects, advantages, and alternatives, willbecome apparent to those of ordinary skill in the art by reading thefollowing detailed description, with reference where appropriate to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method, according to an exampleembodiment.

FIG. 2A is a block diagram illustrating a head mountable displayconfigured to determine eye positions, according to an exampleembodiment.

FIG. 2B shows example glint patterns, according to an exampleembodiment.

FIG. 2C shows an example eye, according to an example embodiment.

FIG. 2D shows an example calibration process, according to an exampleembodiment.

FIG. 3A is a block diagram illustrating a head mountable displayconfigured to determine eye positions, according to an exampleembodiment.

FIG. 3B is a block diagram illustrating another head mountable displayconfigured to determine eye positions, according to an exampleembodiment.

FIG. 3C is a cut-away diagram of an eye, according to an exampleembodiment.

FIG. 4 is a block diagram illustrating a head mountable displayconfigured to determine eye positions and ambient light, according to anexample embodiment.

FIGS. 5A and 5B illustrate a wearable computing device (WCD), accordingto an example embodiment.

FIG. 6 illustrates another wearable computing device, according to anexample embodiment.

FIG. 7 illustrates yet another wearable computing device, according toan example embodiment.

FIG. 8 illustrates an example schematic drawing of a computer networkinfrastructure in which an example embodiment may be implemented.

DETAILED DESCRIPTION

Example methods and systems are described herein. It should beunderstood that the word “exemplary” is used herein to mean “serving asan example, instance, or illustration.” Any embodiment or featuredescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments or features. Theexample embodiments described herein are not meant to be limiting. Itwill be readily understood that certain aspects of the disclosed systemsand methods can be arranged and combined in a wide variety of differentconfigurations, all of which are contemplated herein.

INTRODUCTION

Example systems and methods may be implemented in the context of awearable computer with a head-mounted display (HMD). In particular,example systems and methods involve determining a position of an eyeusing one or more “electromagnetic emitter/sensors” (EESs). The one ormore electromagnetic emitter/sensors can be placed on the head mountabledisplay and configured to emit and detect electromagnetic radiation,a.k.a. light, directed at an eye of a wearer of the head mountabledisplay.

The head wearable display can be configured with electromagneticemitter/sensors for one or both eyes. The electromagneticemitter/sensors can then emit electromagnetic radiation toward one orboth eyes of the wearer and receive indications of receivedelectromagnetic radiation from one or both eyes of the wearer.

After the electromagnetic radiation reflects off of the eye(s), each ofthe electromagnetic emitter/sensors can sense or detect the reflectedelectromagnetic radiation. Upon detecting the reflected electromagneticradiation, each electromagnetic emitter/sensor can determine an amountof received electromagnetic radiation received and generate anindication of a position and/or amount of received electromagneticradiation. Each sensor can provide the indications to a computingdevice, perhaps associated with the head-mountable display.

In most cases, when the emitted electromagnetic radiation reaches aneye, some radiation will be absorbed and some radiation will bereflected. The reflected radiation can form a “glint” or region ofrelatively-high electromagnetic radiation. When a sensor receives thereflected radiation for a glint, an indication of a position, size,area, and/or other data related to the glint can be provided. Theposition of a cornea, which covers the pupil and iris, can be determinedbased on glint patterns, or patterns of glints reflected from an eyefrom radiation emitted by multiple sensors at the same time.

Darker portions of the eye, such as the pupil and iris, tend to absorbmore radiation than lighter portions of the eye, such as the sclera orwhite of the eye. As such, if multiple sensors emit electromagneticradiation toward the eye at the same, or approximately the same time,the sensors that receive the least amount of reflected radiationcorrespond to sensors closest to the darker iris and pupil than tosensors closest to the lighter sclera. In scenarios where sensors areused on both eyes of a wearer, the indications from the two eyes can becombined to determine if both eyes are looking at a same object, areboth closed, or are in different.

The computing device can review all of the indications and determinewhich indication(s) have the lowest amount(s) of receivedelectromagnetic radiation. If an indication from one sensor associatedwith an eye has the lowest amount of received electromagnetic radiation,then the computing device can infer that the sensor is closest to thepupil and iris of the eye. In some cases, the position of theelectromagnetic emitter/sensors can divide a portion of the head-mounteddevice, such as one lens, into sectors or other areas, where each sectorcan be associated with one or more electromagnetic emitter/sensors.After determining which sensor(s) is/are closest to the pupil and irisof the eye, then the computing device can assign the pupil and iris tobe within one or more sectors associated with the closest sensor.

The computing device can then examine indications of receivedelectromagnetic radiation from other sensors, such as neighbors to theclosest sensor, to better estimate the position of the pupil and iris ofthe eye. For example, suppose two sensors have equal or approximatelyequal values of received electromagnetic radiation. Then, the computingdevice can infer that the position of the pupil and iris of the eye maybe nearly equidistant from the two sensors. Once the computing devicehas determined the position of the pupil and iris of the eye, then thecomputing device can determine a gaze direction based on the position ofthe pupil and iris of the eye.

Other portions of the eye can be detected as well. For example, supposeall sensors receive approximately equal amounts of receivedelectromagnetic radiation, and each amount is relatively low. Thecomputing device can then infer the electromagnetic radiation is notbeing reflected from the eye, but perhaps from an eyelid. In this case,by inferring the electromagnetic radiation is reflected off of aneyelid, the computing device can infer that the eye is closed and thatthe wearer is either blinking or has shut their eyes.

Example Methods

FIG. 1 is a flow chart illustrating a method according to an exampleembodiment. Method 100 is described by way of example as being carriedout by a computing device, such as one or more of the wearable computingdevices described herein, but may be carried out by other devices orsystems as well.

Method 100 may be implemented to determine a position of a portion of ahuman eye. Method 100 can begin at block 110.

At block 110, a sensor associated with the computing device can becalibrated in response to an event. In some embodiments, the event caninclude displaying imagery using the computing device. In otherembodiments, calibrating the sensor can include: performing a processusing the computing device. While performing the process, invoking acalibration process and, during the calibration process, calibrating thesensor. In particular embodiments, the process can include a process tolock or unlock the computing device.

At block 120, the sensor associated with the computing device canreceive data indicative of electromagnetic radiation reflected from ahuman eye at the sensor associated with the computing device. Theelectromagnetic radiation, a.k.a. light, can be output by anelectromagnetic emitter/sensor configured to emit light directed towardthe human eye. Once the light is reflected from the eye, theelectromagnetic emitter/sensor can detect the reflected light andgenerate data indicative of electromagnetic radiation reflected from ahuman eye. The electromagnetic emitter/sensor can provide the generateddata indicative of electromagnetic radiation to the computing device. Insome embodiments, the emitted light can be in a pre-determinedfrequency, such as an infrared frequency.

At block 130, the computing device can determine a position of a portionof the human eye based on the received data indicative ofelectromagnetic radiation. The portion of the human eye can include aniris, pupil, and/or eyelid. In some embodiments, a gaze direction of thehuman eye can be determined based on the position of the portion of thehuman eye. In other embodiments, the computing device can determine ifthe human eye is blinking based on the position of the portion of thehuman eye.

At block 140, the computing device can generate an indication thatincludes the position of the portion of the human eye.

At block 150, the computing device can transmit the indication.

Example Head Mountable Displays for Determining Pupil Positions

FIG. 2A shows a right side of head mountable display 200 that includesside-arm 202, center-frame support 204, lens frame 206, lens 208, andelectromagnetic emitter/sensors (EESs) 220 a-220 d. The center framesupport 204 and extending side-arm 202, along with a left extendingside-arm (not shown in FIG. 2A) can be configured to secure head-mounteddevice 200 to a wearer's face via a wearer's nose and ears,respectively. Lens frame 206 can be configured to hold lens 208 at asubstantially uniform distance in front of an eye of the wearer.

Each of electromagnetic emitter/sensors 220 a-220 d can be configured toemit and/or sense electromagnetic radiation in one or more frequencyranges. In one example, each of electromagnetic emitter/sensors 220a-220 d can be configured to emit and sense infrared light. The emittedelectromagnetic radiation can be emitted at one or more specificfrequencies or frequency ranges, such as an infrared frequency, to bothaid detection and to distinguish the emitted radiation from backgroundradiation, such as ambient light. In other embodiments, the emittedelectromagnetic radiation can be emitted using a specific pattern offrequencies or frequency ranges to better distinguish emitted radiationfrom background radiation and to increase the likelihood of detection ofthe emitted radiation after reflection from the eye.

Electromagnetic emitter/sensors 220 a-220 d can be configured to emitelectromagnetic radiation toward a right eye of a wearer of headmountable display 200 and subsequently detect reflected electromagneticradiation to determine a position of a portion of the right eye of thewearer. For example, electromagnetic emitter/sensor 220 a can beconfigured to emit and receive electromagnetic radiation at or near theupper-right-hand portion of the right eye of the wearer, whileelectromagnetic emitter/sensor 220 c can be configured to emit andreceive electromagnetic radiation at or near the lower-left-hand portionof the right eye of the wearer.

For example, suppose at a time T_(A′) the iris and pupil of the righteye of the wearer were located at position A′ shown in FIG. 2A; i.e., atthe center of lens 208. At time T_(A′), electromagnetic emitter/sensors220 a-220 d can emit electromagnetic radiation toward the right eye andthe emitted light can be reflected from the surface of the right eye.Shortly after T_(A′), electromagnetic emitter/sensors 220 a-220 d canreceive the reflected electromagnetic radiation as a “glint” and providedata about the reflected electromagnetic radiation to head mounteddisplay 200 and/or other devices. For example, a sensor ofelectromagnetic emitter/sensors 220 a-220 d receives the reflectedradiation for a glint, an indication of a position, size, area, and/orother data related to the glint can be generated and provided to headmounted display 200 and/or other devices.

A position of the glint can be determined relative to other glintsreceived by electromagnetic emitter/sensors 220 a-220 d to determine arelative direction of an iris and/or pupil of an eye. The iris and pupilof a human eye are covered by the cornea, which is a transparent,dome-shaped structure that bulges outward from the rest of the eyeball.The rest of the eyeball is also covered by a white, opaque layer calledthe sclera. As such, when emitted electromagnetic radiation strikes theeyeball, electromagnetic radiation reflected from the cornea and/orsclera can received at an electromagnetic emitter/sensor.

When electromagnetic radiation reflects from a leading surface of thecornea rather than the sclera (or a trailing surface of the cornea), theelectromagnetic radiation can have less distance to travel before beingreflected. As such, when the cornea is close to a specific sensor, acorresponding glint appears closer to the sensor as well. Also, when thecornea is farther from a specific sensor, a corresponding glint appearsfarther from the sensor.

As the sensors in head mountable display 200 are mounted on lens frame208 near the edges of lens 206, when the cornea is near a closer edge oflens 206, corresponding glints appear closer to the closer edge. Thus, apair of glints reflecting electromagnetic radiation emitted from sensorsmounted on the closer edge appear farther apart than a pair of glintsreflecting electromagnetic radiation emitted from sensors mounted on anedge opposite to the closer edge.

Based on the data about the received reflected electromagneticradiation, a computing device, perhaps associated with head mountabledisplay 200, can determine an estimated position P_(A′) of the iris andpupil of the right eye at T_(A′) is approximately centered within lens208.

FIG. 2B shows example glint pattern 230 that represents glints reflectedfrom a surface of the right eye at position A′. Glint pattern 230 showsfour glints 230 a-230 d respectively corresponding to sensors 220 a-220d. FIG. 2B shows horizontal distance 230 f, which is the shortestdistance between glints 230 a and 230 d; e.g., the top pair of glints inglint pattern 230, as being approximately equal to horizontal distance230 g, which is the shortest distance between glints 230 b and 230 c;e.g., the bottom pair of glints in glint pattern 230. As horizontaldistances 230 f and 230 g are approximately equal, the computing devicecan determine that the cornea, and corresponding pupil and iris, isapproximately horizontally centered between a top horizontal linebetween sensors 220 a and 220 d and a bottom horizontal line betweensensors 220 b and 220 c.

Similarly, FIG. 2B shows vertical distance 230 h, which is the shortestdistance between glints 230 c and 230 d; e.g., the left pair of glintsin glint pattern 230, as being approximately equal to vertical distance230 i, which is the shortest distance between glints 230 a and 230 b;e.g., the right pair of glints in glint pattern 230. As verticaldistances 230 h and 230 i are approximately equal, the computing devicecan determine that the cornea, and corresponding pupil and iris, isapproximately vertically centered between a left vertical line betweensensors 220 c and 220 d and a right vertical line between sensors 220 aand 220 b. Thus, the computing device can determine that the cornea isapproximately both horizontally and vertically centered at T_(A′).

A statistical model to infer eye positions can be developed based onexample glint data. For example, a statistical modeling approach, suchas maximum likelihood estimation, can be used to detail the statisticalmodel. Maximum likelihood estimation takes a given probabilitydistribution model (a.k.a. the statistical model), and a “sample” orcollection of data for the given probability distribution model, andgenerates a parameters for the model that maximize the likelihood of thesample for the given probability distribution model. In someembodiments, the sample can include a number of independent observationsof glints.

A multi-dimensional mapping of sensor data to eye position can be used.For example, each sensor input can act as one dimension of anN-dimensional space, where N=the number of sensors (e.g., N=4 for headmountable display 200). At any given time, the sensors place the eye atone point in the N-dimensional space. Each point in the N-dimensionalspace can be mapped to a two-dimensional eye position. In someembodiments, the mapping can also provide a blink indication of thetwo-dimensional eye position and/or a confidence level of thetwo-dimensional eye position. To generate the N-dimensional totwo-dimensional mapping, the above-mentioned maximum likelihoodestimation technique can be used to map the N-dimensional sensor inputsto a sample of previously recorded two-dimensional calibration data.Other calibration techniques are possible as well; for example,additional calibration techniques are discussed below in the context ofFIG. 2C.

Additionally, the N-dimensional to two-dimensional mapping can be usedas a statistical model to predict eye position. For example, a recursivestate estimation algorithm or similar position-prediction algorithm canuse the N-dimensional to two-dimensional mapping to determine alikelihood of the eye being in a given two-dimensional position based ona previous two-dimensional position of the eye, and thus continuouslymodel the movements of the eye.

As another example, suppose at a time T_(B′), the cornea, including irisand pupil of the right eye of the wearer, is located at position B′shown in FIG. 2A; i.e., relatively near a right edge of lens 208. Acomputing device, perhaps associated with head mountable display 200,can determine an estimated position P_(B′) of the iris and pupil of theright eye at T_(B′) is near the right edge of lens 208.

At time T_(B′), electromagnetic emitter/sensors 220 a-220 d can emitelectromagnetic radiation toward the right eye and the emitted light canbe reflected from the surface of the right eye as a glint pattern. FIG.2B shows example glint pattern 232 that represents glints reflected froma surface of the right eye at position B′. Glint pattern 232 shows fourglints 232 a-232 d respectively corresponding to sensors 220 a-220 d.

FIG. 2B shows vertical distance 232 g, which is the shortest distancebetween glints 232 c and 232 d as being shorter than vertical distance230 f, which is the shortest distance between glints 230 a and 230 b.That is, glints 232 a and 232 b are closer to respective electromagneticemitter/sensors 220 a and 220 b than glints 232 c and 232 d are torespective electromagnetic emitter/sensors 220 c and 220 d. Thecomputing device can utilize this data to determine that the cornea, andcorresponding pupil and iris, is closer to sensors 220 a and 220 b, andthus closer to a right edge of lens 208.

As another example, suppose at a time T_(C′), the cornea, including irisand pupil of the right eye of the wearer, is located at position C′shown in FIG. 2A; i.e., relatively near a top edge of lens 208. Acomputing device, perhaps associated with head mountable display 200,can determine an estimated position P_(C′) of the iris and pupil of theright eye at T_(C′) is near the top edge of lens 208.

At time T_(C′), electromagnetic emitter/sensors 220 a-220 d can emitelectromagnetic radiation toward the right eye and the emitted light canbe reflected from the surface of the right eye as a glint pattern. FIG.2B shows example glint pattern 234 that represents glints reflected froma surface of the right eye at position C′. Glint pattern 234 shows fourglints 234 a-234 d respectively corresponding to sensors 220 a-220 d.

FIG. 2B shows horizontal distance 234 f, which is the shortest distancebetween glints 234 a and 234 d as being longer than horizontal distance234 g, which is the shortest distance between glints 234 b and 234 c.That is, glints 234 a and 234 d are closer to respective electromagneticemitter/sensors 220 a and 220 d than glints 234 b and 234 c are torespective electromagnetic emitter/sensors 220 b and 220 c. Thecomputing device can utilize this data to determine that the cornea, andcorresponding pupil and iris, is closer to sensors 220 a and 220 d, andthus closer to a top edge of lens 208.

As another example, suppose at a time T_(D′), the cornea, including irisand pupil of the right eye of the wearer, is located at position D′shown in FIG. 2A; i.e., relatively near a bottom edge of lens 208. Acomputing device, perhaps associated with head mountable display 200,can determine an estimated position P_(D′) of the iris and pupil of theright eye at T_(D′) is near the bottom edge of lens 208.

At time T_(D′), electromagnetic emitter/sensors 220 a-220 d can emitelectromagnetic radiation toward the right eye and the emitted light canbe reflected from the surface of the right eye as a glint pattern. FIG.2B shows example glint pattern 236 that represents glints reflected froma surface of the right eye at position D′. Glint pattern 236 shows fourglints 236 a-236 d respectively corresponding to sensors 220 a-220 d.

FIG. 2B shows horizontal distance 236 f, which is the shortest distancebetween glints 236 a and 236 d as being shorter than horizontal distance236 g, which is the shortest distance between glints 236 b and 236 c.That is, glints 236 a and 234 d are farther from respectiveelectromagnetic emitter/sensors 220 a and 220 d than glints 236 b and236 c are to respective electromagnetic emitter/sensors 220 b and 220 c.The computing device can utilize this data to determine that the cornea,and corresponding pupil and iris, is closer to sensors 220 b and 220 c,and thus closer to a bottom edge of lens 208.

To ensure accuracy of glint data, sensors 220 a-220 d can be calibratedusing calibration values. In some embodiments, calibration may beperformed when system resources allow; e.g., calibration may notperformed unless a predetermined number of processor cycles per unittime and/or a predetermined amount of idle memory is available forcalibration.

In other embodiments, calibration can be performed in response to anevent, such as displaying a cursor and/or other imagery on lens 208 ofhead mountable display 200. The event may include a prompt; e.g.,“Please look at the cursor to calibrate the display.” In particularembodiments, a calibration process can be invoked as a process orsub-process of a first process. For example, the first process can be aprocess of locking or unlocking a computing device, such as restrictingaccess to or “locking” head mountable display 200 while the owner (orother authorized user) of the head mountable display is notusing/wearing the head mountable display and/or allowing access to or“unlocking” when the owner (or other authorized user) of the headmountable display chooses to use a locked head mountable display 200.

The process of locking/unlocking the computing device can involve a userof head mountable display 200 looking at a pattern of images, such ascolored blocks, perhaps in a specific order. For example, if the patternof images includes one or more images of numeric digits or letters, thenthe ordered pattern of images may spell out a password or pass phrase.As the user looks at each image in order, head mountable display 200 candetermine a likely image the user is likely looking at and can performcalibration between a detected gaze position and an intended gazeposition.

Calibration may involve determining calibration values to align detectedgaze position(s) and intended gaze position(s). For example, if adetected gaze position is at (x_(d), y_(d)) in the plane of lens 208 andan intended gaze position is at (x_(int), y_(int)), then an error(x_(CAL), y_(CAL)) between the intended and detected gaze positions canbe determined as (x_(CAL), y_(CAL))=(x_(int)−x_(d), y_(int)−y_(d)). The(x_(CAL), y_(CAL)) calibration values can be averaged, smoothed,correlated, and/or otherwise processed prior to being used. The(x_(CAL), y_(CAL)) calibration values can be stored as calibrationvalues on a per head-mountable display basis, on a per-sensor basis, oraccording to some other arrangement. After calibration, the (x_(CAL),y_(CAL)) calibration values can be added to the detected gaze positionto get the intended position: (x_(d)+X_(CAL), y_(d)+y_(CAL)). Forexample, if error values are used as (x_(d)+x_(CAL), y_(d)+y_(CAL)),then (x_(d)+x_(CAL), y_(d)+y_(CAL))=(x_(d)+x_(int)−x_(d),y_(d)+y_(int)−y_(d))=(x_(int), y_(int)).

FIG. 2C shows eye 232, according to an example embodiment, shown nearEESs 238 a, 238 b of HMD 238 that are a width w and a depth d from acenter 234 of eye 232. For example, head mountable display 200 shown inFIG. 2A can act as HMD 238. Eye 234 can be looking through cornea 236 ingaze direction 240. Gaze direction 240 touches HMD 238 at gaze point 240a. While gazing, a closest point 244 of cornea 236 to HMD 238 can bedetermined, and a tangent plane 242 including closest point 244 can bedrawn.

As shown in FIG. 2C, tangent plane 242 intersects: a line segment fromcenter 234 to EES 238 a at point V1, gaze direction 240 at point 240 b,and a line segment from center 234 to EES 238 b at point V2. FIG. 2Calso shows reflections G1 and G2 on eye 230 from radiation fromrespective EESs 238 a, 238 b that can be captured as glint data.

FIG. 2D shows calibration process 250, according to an exampleembodiment. Calibration process 250 can begin by obtaining the glintdata, such as glint pattern 260 with pupil position 262, from an eye,such as eye 232, looking at an intended gaze point (x_(int), y_(int)).For example, gaze point (x_(int), y_(int)) can be gaze point 240 a shownin FIG. 2C.

One or more transforms 264 can be performed on the glint data togenerate projected point data 266. For example, a point p₁ in the glintdata can be transformed to a point p₁′ in projected point data by atransform, such as: p₁′=p₁+c₁+c₂(p1−X), where c1, c2 are predeterminedvalues, and X is a predetermined point in the glint data; e.g. such as,but not limited to, a position of pupil 262 or coordinates of apredetermined point; e.g., a central point, upper-left corner point, orlower-right corner point. Other transforms 264 are possible as well. Insome embodiments, transforms 264 may not be used.

FIG. 2D shows five example points in projected point data 266: A, B, C,D, and P. Points A, B, C, and D in projected point data 266 correspond,respectively, to positions of glints a, b, c, and d of glint pattern260. Point P in projected point data 266 corresponds to a position ofpupil 262. Point E can be determined as the intersection of the AC andBD line segments.

FIG. 2D shows that an x-vanishing point Vx can be determined based onprojected point data 266. Vx is determined as the intersection of rays{right arrow over (BC)} and {right arrow over (AD)}. Points M1 and M2can be determined, where M1 is the intersection of line segment AB withray 274. Ray 274 of FIG. 2D starts at x-vanishing point Vx and passesthrough point E. M2 is the intersection of line segment AB with ray 276,where ray 276 starts at Vx and passes through point P as shown in FIG.2D.

Then, an x-cross ratio CR_(x). can be determined. For example, let thecoordinates of A, M1, M2, and B be, respectively, (x1, y1), (x2, y2),(x3, y3), and (x4, y4), as shown in FIG. 2D. Additionally, let D1 be thedistance from A to M2=√{square root over ((x3−x1)²+(y3−y1)²)}, D2 be thedistance from M1 to M2=√{square root over ((x3−x2)²+(y3−y2)²)}, D3 bethe distance from A to B=√{square root over ((x4−x1)²+(y4−y1)²)}, and D4be the distance from M1 to B=√{square root over ((x4−x2)²+(y4−y2)²)}.Then, let the x-cross ratio CR_(x) be CR_(x)=D1/D2/D3/D4.

Further, a y-vanishing point Vy and y-cross ratio CR_(y) can bedetermined. Vy is determined as the intersection of rays {right arrowover (BA)} and {right arrow over (CD)}. Points M3 and M4 can bedetermined, where M3 is the intersection of line segment BC with ray258. Ray 278 of FIG. 2D starts at y-vanishing point Vy and passesthrough point P. M4 is the intersection of line segment BC with ray 280,where ray 280 starts at Vy and passes through point E as shown in FIG.2D.

To determine y-cross ratio CR_(y), let the coordinates of B, M3, M4, andC be, respectively, (x4, y4), (x5, y5), (x6, y6), and (x7, y7), as shownin FIG. 2D. Further, let D5 be the distance from B to M4=√{square rootover ((x6−x4)²+(y6−y4)²)}, D6 be the distance from M3 to M4=√{squareroot over ((x6−x5)²+(y6−y5)²)}, D7 be the distance from B to C=√{squareroot over ((x7−x4)²+(y7−y4)²)}, and D8 be the distance from M3 toC=√{square root over ((x7−x5)²+(y7−y5)²)}. Then, let the y-cross ratioCR_(y) be CR_(y)=D5/D6/D7/D8.

Recall that gaze point 240 a is at coordinates (x_(int), y_(int)). Thecoordinates of the gaze point can be estimated as:x_(int)=w+CR_(x)/1+CR_(x) and y_(int)=d+CR_(y)/1+CR_(y). These estimatedcoordinates can be compared to expected coordinates of a gaze pointbeing observed during calibration process 250. For example, calibrationprocess 250 can take place in response to an event, such as the displayof a cursor and/or other imagery, as discussed above. Then, let(x_(exp), Y_(exp)) be the expected coordinates for a gaze point duringthe event. For example, (x_(exp), Y_(exp)) can be coordinates of acenter point or other point of interest within the cursor and/or otherimagery displayed by HMD 238. Then, error (x_(CAL), y_(CAL)) between theestimated gaze point coordinates (x_(int), y_(int)) and the expectedgaze point coordinates (x_(exp), Y_(exp)) can be calculated in a similarfashion to that discussed above; e.g., (x_(CAL),y_(CAL))=(x_(int)−x_(exp), y_(int)−y_(exp)).

FIG. 3A is a block diagram illustrating head mountable display 300configured to determine eye positions, according to an exampleembodiment. Head mountable display 300 can be implemented as part or allof head mountable displays 200, 320, 402, 502, 602, and 702, and/orcomputing device 802.

FIG. 3A shows a right side of head mountable display 300 that includesside-arm 302, center-frame support 304, lens frame 306, lens 308, andelectromagnetic emitter/sensors (EESs) 310 a-310 c. The center framesupport 304 and extending side-arm 302, along with a left extendingside-arm (not shown in FIG. 3A) can be configured to secure head-mounteddevice 300 to a wearer's face via a wearer's nose and ears,respectively. Lens frame 306 can be configured to hold lens 308 at asubstantially uniform distance in front of an eye of the wearer.

FIG. 3A shows lens 308 divided into sectors 312 a-312 f byelectromagnetic emitter/sensors 310 a-310 c. For example, sector 312 acan be defined by a line from electromagnetic emitter/sensor 310 a to acenter of lens 308, a line from lens frame 306 to the center of lens 308that is equidistant from both electromagnetic emitter/sensor 310 a andelectromagnetic emitter/sensor 310 b, and a portion of lens frame 306between electromagnetic emitter/sensors 310 a and 310 b.

Each of electromagnetic emitter/sensors 310 a-310 c can be configured toemit and/or sense electromagnetic radiation in one or more frequencyranges. In one example, each of electromagnetic emitter/sensors 310a-310 c can be configured to emit and sense infrared light. The emittedelectromagnetic radiation can be emitted at one or more specificfrequencies or frequency ranges, such as an infrared frequency, to bothaid detection and to distinguish the emitted radiation from backgroundradiation, such as ambient light. In other embodiments, the emittedelectromagnetic radiation can be emitted using a specific pattern offrequencies or frequency ranges to better distinguish emitted radiationfrom background radiation and to increase the likelihood of detection ofthe emitted radiation after reflection from the eye.

Electromagnetic emitter/sensors 310 a-310 c can be configured to emitelectromagnetic radiation toward a right eye of a wearer of headmountable display 300 and subsequently detect reflected electromagneticradiation to determine a position of a portion of the right eye of thewearer. For example, electromagnetic emitter/sensor 310 a can beconfigured to emit and receive electromagnetic radiation at or near thetop of the right eye of the wearer, while electromagnetic emitter/sensor310 c can be configured to emit and receive electromagnetic radiation ator near the left side of the right eye of the wearer.

For example, suppose at a time T_(A) the iris and pupil of the right eyeof the wearer were located at position A shown in FIG. 3A; i.e., at thecenter of lens 308. At time T_(A), let electromagnetic emitter/sensors310 a-310 c emit electromagnetic radiation toward the eye of the wearer,where the emitted light can be reflected from the surface of the eye.Shortly after T_(A), electromagnetic emitter/sensors 310 a-310 c canreceive the reflected electromagnetic radiation and provide data aboutthe reflected electromagnetic radiation to head mounted display 300and/or other devices.

Based on the data about the received reflected electromagneticradiation, a computing device, perhaps associated with head mountabledisplay 300, can determine an estimated position P_(A) of the iris andpupil of the right eye at T_(A). In this example, the reflected light ateach of electromagnetic emitter/sensors 310 a-310 c is approximately thesame, as each sector of lens 308 is equally covered by the iris andpupil of the right eye.

As another example, suppose at a time T_(B) the iris and pupil of theright eye of the wearer were located at position B shown in FIG. 3A;primarily in sector 312 b toward the upper-rightmost portion of lens308. At time T_(B), let electromagnetic emitter/sensors 310 a-310 c emitelectromagnetic radiation toward the eye of the wearer, where theemitted light can be reflected from the surface of the eye. Shortlyafter T_(B), electromagnetic emitter/sensors 310 a-310 c can receive thereflected electromagnetic radiation.

In this example, suppose the amounts of received light at each ofelectromagnetic emitter/sensors 310 a-310 c as shown in Table 1 below:

TABLE 1 Sensor(s) Amount of Received Light (on 1-10 Scale) at T_(B) 310a4 310b 3 310c 9

In other embodiments, one or more of sensors 310 a-310 c can providemore or less information about received light to wearable computingdevice 300. As one example, the amount of received light can beexpressed using either a single bit, with 0 being dark and 1 beinglight. As another example, a scale finer than a 1 to 10 scale can beused; e.g., a 0 (dark) to 255 (bright) scale. Additionally or instead,information about frequencies, direction, and/or other features ofreceived light can be provided by one or more of sensors 310 a-310 c.Upon receiving the light, each of sensors 310 a-310 c can determine theamount of received light, generate an indication of the amount ofreceived light using one or more of the numerical scales, and providethe indication to head mountable display 300.

Based on the information about received reflected electromagneticradiation, a computing device, perhaps associated with head mountabledisplay 300, can determine an estimated position of the iris and pupilP_(B) of the right eye at T_(B). As the amount of reflected light atsensor 310 c is relatively large and the amounts of reflected light atsensors 310 b and 310 c are relatively small, head mountable display 300can determine that there is a relatively-large probability that P_(B) iswithin either sector 312 a or 312 b. Then, considering that thereflected light at sensor 310 a is slightly higher (4 out of 10) than atsensor 310 b (3 out of 10), head mountable display 300 can determinethat there is a relatively-smaller probability that P_(B) is withinsector 312 a than P_(B) is within sector 312 b, and can maintain orperhaps increase a probability that P_(B) is on a boundary betweensector 312 a and 312 b.

Additionally, suppose at a time T_(C) the iris and pupil of the righteye of the wearer were located at position C in sector 312 e at anearly-leftmost portion of lens 308. At time T_(C), let electromagneticemitter/sensors 310 a-310 c emit electromagnetic radiation toward theeye of the wearer, where the emitted light can be reflected from thesurface of the eye. Shortly after T_(C), electromagnetic emitter/sensors310 a-310 c can receive the reflected electromagnetic radiation.

In this example, suppose the amounts of received light at each ofelectromagnetic emitter/sensors 310 a-310 c as shown in Table 2 below:

TABLE 2 Sensor(s) Amount of Received Light (on 1-10 Scale) at T_(C) 310a4 310b 9 310c 2

Based on the received reflected electromagnetic radiation, a computingdevice, perhaps associated with head mountable display 300, candetermine an estimated position of the iris and pupil P_(C) of the righteye at T_(C). As the amount of reflected light at sensor 310 b isrelatively large and the amount of reflected light at sensors 310 a and310 c is relatively small, head mountable display 300 can determine thatthere is a relatively-large probability that P_(C) is within eithersector 312 e or 312 f. Then, considering that the reflected light atsensor 310 a is higher (4 out of 10) than at sensor 310 b (2 out of 10),head mountable display 300 can determine that there is arelatively-smaller probability that P_(C) is within sector 312 f thanP_(C) is within sector 312 e, and can perhaps decrease a probabilitythat P_(C) is on a boundary between sector 312 e and 312 f.

As another example, suppose at a time T_(D) the iris and pupil of theright eye of the wearer were located at position D on the sector 312c/312 d boundary at a lower portion of lens 308. At time T_(D), letelectromagnetic emitter/sensors 310 a-310 c emit electromagneticradiation toward the eye of the wearer, where the emitted light can bereflected from the surface of the eye. Shortly after T_(D),electromagnetic emitter/sensors 310 a-310 c can receive the reflectedelectromagnetic radiation.

In this example, suppose the amounts of received light at each ofelectromagnetic emitter/sensors 310 a-310 c are as shown in Table 3below:

TABLE 3 Sensor(s) Amount of Received Light (on 1-10 Scale) at T_(D) 310a6 310b 4 310c 4

Based on the received reflected electromagnetic radiation, a computingdevice, perhaps associated with head mountable display 300, candetermine an estimated position of the iris and pupil P_(D) of the righteye at T_(D). As the amount of reflected light at sensor 310 a isrelatively large and the amount of reflected light at sensors 310 b and310 c is relatively small, head mountable display 300 can determine thatthere is a relatively-large probability that P_(C) is within eithersector 312 c or 312 d. Then, considering that the reflected light atsensor 310 b is equal to that at sensor 310 b (both 4 out of 10), headmountable display 300 can determine that there is a relatively-largeprobability that a P_(D) is on a boundary between sector 312 c and 312d.

In other embodiments, electromagnetic emitter/sensors 310 a-310 c caneach be configured to emit electromagnetic radiation using part or allof the visible frequency range, the infrared frequency range, theultraviolet frequency range, and/or another frequency range e.g.,microwave, X-ray, etc. Also, each of electromagnetic emitter/sensors 310a-310 c can be configured to detect electromagnetic radiation in one ormore frequency ranges.

In still other embodiments, some or all of electromagneticemitter/sensors 310 a-310 c can be configured to detect electromagneticradiation only; e.g., act as a camera. In still other embodiments, headmountable display 300 can be configured with more or fewerelectromagnetic emitter/sensors 310 a-310 c than shown in FIG. 3A.

Other portions of the eye can be detected as well. For example, supposeeach of electromagnetic emitter/sensors 310 a-310 c receiveapproximately equal amounts of received electromagnetic radiation, andeach amount is relatively low. Then a computing device, perhaps part ofhead mountable display 300, can infer the electromagnetic radiation isnot being reflected from the eye, but from an eyelid. In this case, byinferring the electromagnetic radiation is reflected from an eyelid, thecomputing device can infer that the eye is closed and that the wearer iseither blinking (closed the eye for a short time) or has shut their eyes(closed the eye for a longer time).

To determine if the wearer is blinking or has shut their eyes, thecomputing device can wait a predetermined amount of time, and thenrequest a second set of indications of reflected electromagneticradiation from the electromagnetic emitter/sensors.

The predetermined amount of time can be based on a blink duration and/ora blink interval. In adult humans, a blink duration, or how long the eyeis closed during a blink is approximately 300-400 milliseconds, and ablink rate, or how often the eye is blinked under typical conditions, isbetween two and ten blinks per minute; i.e., one blink per every six tothirty seconds.

For example, let the pre-determined amount of time be longer than ablink interval but less than a blink duration. Then, if the indicationsof reflected electromagnetic radiation were taken from a closed eyeduring a blink, then the requested second set of indications ofreflected electromagnetic radiation taken after the pre-determinedamount of time should be from an open eye, if the wearer were blinking.If the second set of second set of indications of reflectedelectromagnetic radiation were taken from a closed eye as well,additional sets of indications of received electromagnetic radiation canbe generated, perhaps at random intervals of time, to determine whetherthe eye is closed by receiving additional sets of indications withrelatively low and uniform amounts of received electromagneticradiation, or whether the eye is now open and determine if a blink wasdetected.

Using additional sets of indications of received electromagneticradiation taken from another eye of the wearer can determine if thewearer has both eyes open, both eyes closed, or has one eye open; e.g.,is winking. Also, indications of received electromagnetic radiationtaken from another eye can be used to confirm values of receivedelectromagnetic radiation when both eyes can be inferred to be closed.

Other techniques can be used to determine a position of an eye beyondthose described herein.

FIG. 3B is a block diagram illustrating head mountable display 330configured to determine eye positions, according to an exampleembodiment. Head mountable display 330 can be implemented as part or allof head mountable displays 300, 402, 502, 602, and 702, and/or computingdevice 802.

FIG. 3B shows a right side of head mountable display 330 that includesside-arm 302, center-frame support 304, lens frame 306, lens 308, andelectromagnetic emitter/sensors (EESs) 320 a-320 e. Side-arm 302, centerframe support 304, and lens frame 306 are as discussed above in thecontext of FIG. 3A.

FIG. 3B shows electromagnetic emitter/sensor 320 e at the center of lens308. Electromagnetic emitter/sensor 320 e can be embedded within lens308 and/or be a component of a layer of material overlaying part or allof lens 308. In some embodiments, electromagnetic emitter/sensor 320 eis configured to be invisible or nearly invisible to the human eye. Inother embodiments, electromagnetic emitter/sensor 320 e is not locatedat the center of lens 308. In still other embodiments, multipleelectromagnetic emitter/sensors are embedded within lens 308 and/or be acomponent of a layer of material overlaying part or all of lens 308.

Lens 308 is shown in FIG. 3B as divided into sectors 322 a-322 i byelectromagnetic emitter/sensors 320 a-320 e. In particular, sector 322 ais defined by the circle whose radius is one half of a distance from thecenter of lens 308, where electromagnetic emitter/sensor 320 e islocated, to each of electromagnetic emitter/sensors 320 a-320 d.

As another example, sector 322 b is defined by a line fromelectromagnetic emitter/sensor 310 a to a center of lens 308, anupper-right portion of the sector boundary of sector 322 a, a line fromlens frame 306 to the center of lens 308 that is equidistant from bothelectromagnetic emitter/sensor 320 a and electromagnetic emitter/sensor320 b, and a portion of lens frame 306 between electromagneticemitter/sensors 320 a and 320 b.

Each of electromagnetic emitter/sensors 320 a-320 e can be configured toemit and/or sense electromagnetic radiation in one or more frequencyranges, as discussed above in the context of FIG. 3A. Each ofelectromagnetic emitter/sensors 310 a-310 c can be configured to emitelectromagnetic radiation toward a right eye of a wearer of headmountable display 300 and subsequently detect reflected electromagneticradiation to determine a position of a portion of the right eye of thewearer, as discussed above in the context of FIG. 3A.

For example, suppose at a time T2 _(A) the iris and pupil of the righteye of the wearer were located at position A shown in FIG. 3A; i.e., atthe center of lens 308. At time T2 _(A), let electromagneticemitter/sensors 320 a-320 e emit electromagnetic radiation toward theeye of the wearer, where the emitted light can be reflected from thesurface of the eye. Shortly after T2 _(A), electromagneticemitter/sensors 320 a-320 e can receive the reflected electromagneticradiation and provide data about the reflected electromagnetic radiationto head mounted display 330 and/or other devices.

Based on the data about received reflected electromagnetic radiation, acomputing device, perhaps associated with head mountable display 330,can determine a position of the iris and pupil of the right eye at T2_(A). In this example, the reflected light received at electromagneticemitter/sensor 320 e is relatively low and the reflected light at eachof electromagnetic emitter/sensors 320 a-320 d is approximately the sameand brighter than the reflected light received at sensor 320 e. Thisobservation can be due to the iris and pupil of the right eye being veryclose to sensor 320 e and being approximately equidistant and fartherfrom each of each of electromagnetic emitter/sensors 320 a-320 d.

As another example, suppose at a time T_(E) the iris and pupil of theright eye of the wearer were located at position E shown in FIG. 3B;primarily in sector 322 c with some overlap into sector 322 a toward theupper-rightmost portion of lens 308. At time T_(E), let electromagneticemitter/sensors 320 a-320 e emit electromagnetic radiation toward theeye of the wearer, where the emitted light can be reflected from thesurface of the eye. Shortly after T_(E), electromagnetic emitter/sensors320 a-320 e can receive the reflected electromagnetic radiation.

In this example, suppose the amounts of received light at each ofelectromagnetic emitter/sensors 320 a-320 e as shown in Table 4 below:

TABLE 4 Sensor(s) Amount of Received Light (on 1-10 Scale) at T_(E) 320a4 320b 2 320c 8 320d 9 320e 3

In other embodiments, one or more of sensors 320 a-320 e can providemore or less information about received light, as discussed above in thecontext of FIG. 3A.

Based on the information about received reflected electromagneticradiation, a computing device, perhaps associated with head mountabledisplay 330, can determine an estimated position of the iris and pupilP_(E) of the right eye at T_(E). As the amounts of reflected light atsensors 320 c and 320 d are relatively large and the amounts ofreflected light at sensors 320 a, 320 b and 320 e is relatively small,head mountable display 330 can determine that there is arelatively-large probability that P_(B) is in the upper-right quarter oflens 308 within either the upper-right portion of sector 322 a, insector 322 b or in sector 322 c. Then, considering that the reflectedlight at sensor 320 a is slightly higher (4 out of 10) than at sensor320 e (3 out of 10), and that the reflected light at sensor 320 b (2 outof 10) is slightly lower than at sensor 320 e, head mountable display330 can determine that P_(B) is (in decreasing orders of likelihood)either: (1) on a boundary between sector 322 c and sector 322 a withinsector 322 c, (2) within sector 322 a, (3) on a boundary between sector322 a and 322 b, (4) on a boundary between sector 322 a and 322 b, or(5) within sector 322 b.

In embodiments not shown in either FIG. 3A or FIG. 3B, a left side ofhead mountable display 300 or 330 can be configured with a side-arm,lens-support, lens, and electromagnetic emitter/sensors, and connectedto the right side of head mountable display 300 or 320 via center-framesupport 304. In these embodiments, the left side of head mountabledisplay 300 and/or 330 can be configured with electromagneticemitter/sensors for a left eye of the wearer.

FIG. 3C is a cut-away diagram of eye 340, according to an exampleembodiment. FIG. 3C shows retina 342 at the back of eye 340 within eyesocket 344. Sectors 222 a and 222 f-222 i are projected onto a frontsurface of eye 340. FIG. 3C shows a pupil 346 of eye 340 in a positionG., which corresponds to position G of FIG. 3B. As shown in both FIGS.3B and 2C, pupil 346 at position G is in sector 222 i while also beingclose to the boundary with sector 222 h.

Preliminary gaze direction 350 of eye 340 can be determined based on adetermined position of pupil 346. Pupil 346 can be determined to be inposition G, perhaps using the techniques discussed above in the contextof FIGS. 3A and 3B. Then, preliminary gaze direction 350 can bedetermined as a line segment starting at retina 342 and passing throughpupil 346. As pupil 346 has been determined to be at position G,preliminary gaze direction 350 passes through position G as well. Thus,preliminary gaze direction 350 can be determined based on the determinedposition G of pupil 346.

In some embodiments, a head-position vector or other data related tohead movement can be determined using one or more accelerometers orother sensors configured to detect head movement. For example,head-mounted device 300, 330, 402, 502, 602, and/or 702 can equippedwith a sensor such as sensor 422 that is configured to generate ahead-position vector indicating a tilt of the head.

Then, a computing device, such as computing device 802, that can beassociated with head-mounted device 300, 330, 402, 502, 602, and/or 702can combine the head-position vector, indicating head tilt, and thepreliminary gaze direction, indicating eye gaze, to generate a finalgaze direction that indicates the eye gaze after taking head tilt intoconsideration. For example, suppose the head-position vector andpreliminary gaze direction 350 are both vectors in the same coordinatespace. Then, the final gaze direction can be determined, at least inpart, by performing vector addition on the head-position vector and thepreliminary gaze direction vector. Other techniques for generatingpreliminary gaze directions, head-position vectors, and final gazedirections are possible as well.

FIG. 4 is a block diagram illustrating a head mountable display 402configured to determine pupil positions and ambient light, according toan example embodiment. Head mountable display 402 can be implemented aspart or all of head mountable displays 300, 330, 502, 602, and 702,and/or computing device 802.

Head mountable display 402 includes the components of head mountabledisplay 330 as shown in FIG. 3B as well as ambient light sensors (ALSs)410 a-410 d. Each of ambient light sensors 410 a-410 d is respectivelylocated near electromagnetic emitter/sensors 320 a-320 d and isconfigured to detect ambient light and output an indication of theamount of the detected ambient light. The indication can be binary;e.g., 0 indicating no ambient light and 1 indicating some ambient light,numeric, such as using a 0 (dark) to 10 (bright) scale or as apercentage, qualitative, such as “dark”, “dim”, “bright”, and “verybright”, a combination of binary, numeric, qualitative, e.g., dim=15 outof 255 possible, or using some other technique for indicating an amountof ambient light.

As shown in FIG. 4, at a time T_(H), light source 420 is located abovethe right side-arm 202 of head mountable display 402 and is emittingambient light 422. In other scenarios, light source(s) can provide lessor more ambient light; while in other scenarios, no ambient light may bepresent.

FIG. 4 also shows that, at T_(H), an iris and pupil of an eye of awearer of head mountable display 402 is at position H, which is at theboundary of sectors 222 a, 222 f, and 222 g. At time T_(H), letelectromagnetic emitter/sensors 210 a-210 c emit electromagneticradiation toward the eye of the wearer, where the emitted light can bereflected from the surface of the eye. Shortly after T_(H),electromagnetic emitter/sensors 210 a-210 c can receive the reflectedelectromagnetic radiation.

In this example, suppose the amounts of received light at each ofelectromagnetic emitter/sensors 210 a-210 c and the amount of ambientlight received at each of sensors 410 a-410 d are as shown in Table 5below:

TABLE 5 Received Light Ambient Light Sensors (on 1-10 Scale) at T_(H)(1-10 Scale) at T_(H) 320a/410a 9 10  320b/410b 9 8 320c/410c 4 6320d/410d 4 7 320e 4 n/a

A number of electromagnetic emitter/sensors used to detect emittedelectromagnetic radiation can be determined based on a detected amountof ambient light. For example, using the data shown in Table 5 above,all four ambient light sensors 410 a-410 d indicate at least a moderatelevel of ambient light and sensor 410 a indicates a maximum amount ofambient light. In these conditions with moderate to high amounts ofdetected ambient light, all of electromagnetic emitter/sensors 320 a-320e can be used to increase the likelihood that the emittedelectromagnetic radiation is being detected and not being overshadowedby the ambient light

In contrast, where there is little or no detected ambient light, fewerelectromagnetic emitter/sensors can be used to emit electromagneticradiation. In these low-light conditions, as the likelihood the emittedelectromagnetic radiation from one or a few electromagneticemitter/sensors is detected and not overcome the (small) amount ofambient light is reasonably high. In some embodiments, electromagneticemitter/sensors not emitting electromagnetic radiation can still detectthe emitted electromagnetic radiation from the other emitting sensor(s).In other embodiments, electromagnetic emitter/sensors not emittingelectromagnetic radiation can be turned off and so do not detect theemitted electromagnetic radiation from the other emitting sensor(s).

An electromagnetic emitter/sensor can change emission of electromagneticradiation based on a detected amount of ambient light. For example,using the numbers in Table 5 above, ambient light sensor 410 a mayprovide an indication to electromagnetic emitter/sensor 320 a thatambient light at level 10 of 10 was received. In response,electromagnetic emitter/sensor 320 a can change an amount of power,time, wavelength(s), and/or other properties of emitted electromagneticradiation to be emitted toward an eye of the wearer; for example,electromagnetic emitter/sensor 320 a can determine that the ambientlight is bright and therefore increase power and/or an amount of timefor emitting radiation toward an eye of the wearer, or perhaps change afrequency of emitted radiation to use a frequency not commonly found inambient light.

As another example, using the numbers in Table 5 above, ambient lightsensor 410 c may provide an indication to electromagnetic emitter/sensor320 c that ambient light at level 6 of 10 was received. In response,electromagnetic emitter/sensor 320 a can determine that the amount ofambient light is relatively moderate and either maintain or change anamount of power, time, wavelength(s), and/or other properties of emittedelectromagnetic radiation to be emitted toward an eye of the wearer; forexample, electromagnetic emitter/sensor 320 a can determine that theambient light is moderate and therefore maintain power and/or an amountof time for emitting radiation toward an eye of the wearer. Otherexamples are possible as well.

FIG. 4 also shows that electromagnetic emitter/sensor 320 e does nothave an associated ambient light sensor. In some embodiments not shownin FIG. 4, head mountable display 402 can be configured with one or moreambient light sensors that are not located near electromagneticemitters/sensors.

FIG. 4 shows that an electromagnetic emitter/sensor is a separatecomponent from an ambient light sensor component. In some embodimentsnot shown in FIG. 4, an electromagnetic emitter/sensor component caninclude the functionality of an ambient light sensor.

Example Systems and Devices

Systems and devices in which example embodiments may be implemented willnow be described in greater detail. In general, an example system may beimplemented in or may take the form of a wearable computer. However, anexample system may also be implemented in or take the form of otherdevices, such as a mobile phone, among others. Further, an examplesystem may take the form of non-transitory computer readable medium,which has program instructions stored thereon that are executable by ata processor to provide the functionality described herein. An example,system may also take the form of a device such as a wearable computer ormobile phone, or a subsystem of such a device, which includes such anon-transitory computer readable medium having such program instructionsstored thereon.

FIGS. 5A and 5B illustrate a wearable computing device 500, according toan example embodiment. In FIG. 5A, the wearable computing device 500takes the form of a head-mountable device (HMD) 502 (which may also bereferred to as a head-mountable display). It should be understood,however, that example systems and devices may take the form of or beimplemented within or in association with other types of devices,without departing from the scope of the invention.

As illustrated in FIG. 5A, the head-mountable device 502 comprises frameelements including lens-frames 504 and 506 and a center frame support508, lens elements 510 and 512, and extending side-arms 514 and 516. Thecenter frame support 508 and the extending side-arms 514 and 516 areconfigured to secure the head-mountable device 502 to a wearer's facevia a wearer's nose and ears, respectively.

Each of the frame elements 504, 506, and 508 and the extending side-arms514 and 516 may be formed of a solid structure of plastic or metal, ormay be formed of a hollow structure of similar material so as to allowwiring and component interconnects to be internally routed through thehead-mountable device 502. Other materials may possibly be used as well.

One or both of lens elements 510 and 512 may be formed of any materialthat can suitably display a projected image or graphic. One or both oflens elements 510 and 512 may also be sufficiently transparent to allowa wearer to see through the lens element. Combining these two featuresof lens elements 510, 512 can facilitate an augmented reality orheads-up display where the projected image or graphic is superimposedover a real-world view as perceived by the wearer through the lenselements.

The extending side-arms 514 and 516 each may be projections that extendaway from the frame elements 504 and 506, respectively, and arepositioned behind a wearer's ears to secure the head-mountable device502. The extending side-arms 514 and 516 may further secure thehead-mountable device 502 to the wearer by extending around a rearportion of the wearer's head. Additionally or alternatively, forexample, head-mountable device 502 may connect to or be affixed within ahead-mounted helmet structure. Other possibilities exist as well.

Head-mountable device 502 may also include an on-board computing system518, video camera 520, sensor 522, and finger-operable touchpads 524,526. The on-board computing system 518 is shown on the extendingside-arm 514 of the head-mountable device 502; however, the on-boardcomputing system 518 may be positioned on other parts of thehead-mountable device 502 or may be remote from head-mountable device502; e.g., the on-board computing system 518 could be wired to orwirelessly-connected to the head-mounted device 502.

The on-board computing system 518 may include a processor and memory,for example. The on-board computing system 518 may be configured toreceive and analyze data from video camera 520, sensor 522, and thefinger-operable touchpads 524, 526 (and possibly from other sensorydevices, user interfaces, or both) and generate images for output fromthe lens elements 510 and 512 and/or other devices.

The sensor 522 is shown mounted on the extending side-arm 516 of thehead-mountable device 502; however, the sensor 522 may be provided onother parts of the head-mountable device 502. The sensor 522 may includeone or more of a gyroscope or an accelerometer, for example. Othersensing devices may be included within the sensor 522 or other sensingfunctions may be performed by the sensor 522.

In an example embodiment, sensors such as sensor 522 may be configuredto detect head movement by a wearer of head-mountable device 502. Forinstance, a gyroscope and/or accelerometer may be arranged to detecthead movements, and may be configured to output head-movement data. Thishead-movement data may then be used to carry out functions of an examplemethod, such as method 100, for instance.

The finger-operable touchpads 524, 526 are shown mounted on theextending side-arms 514, 516 of the head-mountable device 502. Each offinger-operable touchpads 524, 526 may be used by a wearer to inputcommands. The finger-operable touchpads 524, 526 may sense at least oneof a position and a movement of a finger via capacitive sensing,resistance sensing, or a surface acoustic wave process, among otherpossibilities. The finger-operable touchpads 524, 526 may be capable ofsensing finger movement in a direction parallel or planar to the padsurface, in a direction normal to the pad surface, or both, and may alsobe capable of sensing a level of pressure applied. The finger-operabletouchpads 524, 526 may be formed of one or more transparent ortransparent insulating layers and one or more transparent or transparentconducting layers. Edges of the finger-operable touchpads 524, 526 maybe formed to have a raised, indented, or roughened surface, so as toprovide tactile feedback to a wearer when the wearer's finger reachesthe edge of the finger-operable touchpads 524, 526. Each of thefinger-operable touchpads 524, 526 may be operated independently, andmay provide a different function.

FIG. 5B illustrates an alternate view of the wearable computing deviceshown in FIG. 5A. As shown in FIG. 5B, the lens elements 510 and 512 mayact as display elements. The head-mountable device 502 may include afirst projector 528 coupled to an inside surface of the extendingside-arm 516 and configured to project a display 530 onto an insidesurface of the lens element 512. Additionally or alternatively, a secondprojector 532 may be coupled to an inside surface of the extendingside-arm 514 and configured to project a display 534 onto an insidesurface of the lens element 510.

The lens elements 510 and 512 may act as a combiner in a lightprojection system and may include a coating that reflects the lightprojected onto them from the projectors 528 and 532. In someembodiments, a special coating may not be used (e.g., when theprojectors 528 and 532 are scanning laser devices).

In alternative embodiments, other types of display elements may also beused. For example, the lens elements 510, 512 themselves may include: atransparent or semi-transparent matrix display, such as anelectroluminescent display or a liquid crystal display, one or morewaveguides for delivering an image to the wearer, or other opticalelements capable of delivering an in focus near-to-eye image to thewearer. A corresponding display driver may be disposed within the frameelements 504 and 506 for driving such a matrix display. Alternatively oradditionally, a laser or light-emitting diode (LED) source and scanningsystem could be used to draw a raster display directly onto the retinaof one or more of the wearer's eyes. Other possibilities exist as well.

While FIGS. 5A and 5B show two touchpads and two display elements, itshould be understood that many example methods and systems may beimplemented in wearable computing devices with only one touchpad and/orwith only one lens element having a display element. It is also possiblethat example methods and systems may be implemented in wearablecomputing devices with more than two touchpads.

The outward-facing video camera 520 is shown to be positioned on theextending side-arm 514 of the head-mountable device 502; however, theoutward-facing video camera 520 may be provided on other parts of thehead-mountable device 502. The outward-facing video camera 520 may beconfigured to capture images at various resolutions or at differentframe rates. Many video cameras with a small form-factor, such as thoseused in cell phones or webcams, for example, may be incorporated into anexample of wearable computing device 500.

Although FIG. 5A illustrates one outward-facing video camera 520, moreoutward-facing video cameras may be used than shown in FIG. 5A, and eachoutward-facing video camera may be configured to capture the same view,or to capture different views. For example, the outward-facing videocamera 520 may be forward facing to capture at least a portion of thereal-world view perceived by the wearer. This forward facing imagecaptured by the outward-facing video camera 520 may then be used togenerate an augmented reality where computer generated images appear tointeract with the real-world view perceived by the wearer.

In some embodiments not shown in FIGS. 5A and 5B, wearable computingdevice 500 can also or instead include one or more inward-facingcameras. Each inward-facing camera can be configured to capture stillimages and/or video of part or all of the wearer's face. For example,the inward-facing camera can be configured to capture images of an eyeof the wearer. Wearable computing device 500 may use other types ofsensors to detect a wearer's eye movements, in addition to or in thealternative to an inward-facing camera. For example, wearable computingdevice 500 could incorporate a proximity sensor or sensors, which may beused to measure distance using infrared reflectance. In one suchembodiment, lens element 510 and/or 512 could include a number of LEDswhich are each co-located with an infrared receiver, to detect when awearer looks at a particular LED. As such, eye movements between LEDlocations may be detected. Other examples are also possible.

FIG. 6 illustrates another wearable computing device, according to anexample embodiment, which takes the form of head-mountable device 602.Head-mountable device 602 may include frame elements and side-arms, suchas those described with respect to FIGS. 5A and 5B. Head-mountabledevice 602 may additionally include an on-board computing system 604 andvideo camera 606, such as described with respect to FIGS. 5A and 5B.Video camera 606 is shown mounted on a frame of head-mountable device602. However, video camera 606 may be mounted at other positions aswell.

As shown in FIG. 6, head-mountable device 602 may include display 608which may be coupled to a wearable computing device. Display 608 may beformed on one of the lens elements of head-mountable device 602, such asa lens element described with respect to FIGS. 5A and 5B, and may beconfigured to overlay computer-generated graphics on the wearer's viewof the physical world.

Display 608 is shown to be provided in a center of a lens ofhead-mountable device 602; however, the display 608 may be provided inother positions. The display 608 can be controlled using on-boardcomputing system 604 coupled to display 608 via an optical waveguide610.

FIG. 7 illustrates yet another wearable computing device, according toan example embodiment, which takes the form of head-mountable device702. Head-mountable device 702 can include side-arms 723, a center framesupport 724, and a bridge portion with nosepiece 725. In the exampleshown in FIG. 7, the center frame support 724 connects the side-arms723. As shown in FIG. 7, head-mountable device 702 does not includelens-frames containing lens elements. Head-mountable device 702 mayadditionally include an on-board computing system 726 and video camera728, such as described with respect to FIGS. 5A and 5B.

Head-mountable device 702 may include a single lens element 730configured to be coupled to one of the side-arms 723 and/or center framesupport 724. The lens element 730 may include a display such as thedisplay described with reference to FIGS. 5A and 5B, and may beconfigured to overlay computer-generated graphics upon the wearer's viewof the physical world. In one example, the single lens element 730 maybe coupled to the inner side (i.e., the side exposed to a portion of awearer's head when worn by the wearer) of the extending side-arm 723.The single lens element 730 may be positioned in front of or proximateto a wearer's eye when head-mountable device 702 is worn. For example,the single lens element 730 may be positioned below the center framesupport 724, as shown in FIG. 7.

FIG. 8 illustrates a schematic drawing of a computing system 800according to an example embodiment. In system 800, a computing device802 communicates using a communication link 810 (e.g., a wired orwireless connection) to a remote device 820. Computing device 802 may beany type of device that can receive data and display informationcorresponding to or associated with the data. For example, the device802 may be associated with and/or be part or all of a heads-up displaysystem, such as the head-mounted devices 300, 330, 402, 502, 602, and/or702 described with reference to FIGS. 3A-6.

Thus, computing device 802 may include display system 830 comprisingprocessor 840 and a display 850. Display 850 may be, for example, anoptical see-through display, an optical see-around display, or a videosee-through display. Processor 840 may receive data from remote device820 and configure the data for display on display 850. Processor 840 maybe any type of processor, such as a micro-processor or a digital signalprocessor, for example.

Computing device 802 may further include on-board data storage, such asmemory 860 coupled to the processor 840. Memory 860 may store softwarethat can be accessed and executed by the processor 840. For example,memory 860 may store software that, if executed by processor 840 isconfigured to perform some or all of the functionality described herein,for example.

Remote device 820 may be any type of computing device or transmitterincluding a laptop computer, a mobile telephone, or tablet computingdevice, etc., that is configured to transmit and/or receive data to/fromcomputing device 802. Remote device 820 and computing device 802 maycontain hardware to establish, maintain, and tear down communicationlink 810, such as processors, transmitters, receivers, antennas, etc.

In FIG. 8, communication link 810 is illustrated as a wirelessconnection; however, communication link 810 can also or instead includewired connection(s). For example, the communication link 810 may includea wired serial bus such as a universal serial bus or a parallel bus. Awired connection may be a proprietary connection as well. Thecommunication link 810 may also include a wireless connection using,e.g., Bluetooth® radio technology, communication protocols described inIEEE 802.11 (including any IEEE 802.11 revisions), cellular technology(such as GSM, CDMA, UMTS, EV-DO, WiMAX, or LTE), or Zigbee® technology,among other possibilities. Computing device 802 and/or remote device 820may be accessible via the Internet and may include a computing clusterassociated with a particular web service (e.g., social-networking, photosharing, address book, etc.).

CONCLUSION

The above detailed description describes various features and functionsof the disclosed systems, devices, and methods with reference to theaccompanying figures. In the figures, similar symbols typically identifysimilar components, unless context dictates otherwise. The illustrativeembodiments described in the detailed description, figures, and claimsare not meant to be limiting. Other embodiments can be utilized, andother changes can be made, without departing from the spirit or scope ofthe subject matter presented herein. It will be readily understood thatthe aspects of the present disclosure, as generally described herein,and illustrated in the figures, can be arranged, substituted, combined,separated, and designed in a wide variety of different configurations,all of which are explicitly contemplated herein.

With respect to any or all of the ladder diagrams, scenarios, and flowcharts in the figures and as discussed herein, each block and/orcommunication may represent a processing of information and/or atransmission of information in accordance with example embodiments.Alternative embodiments are included within the scope of these exampleembodiments. In these alternative embodiments, for example, functionsdescribed as blocks, transmissions, communications, requests, responses,and/or messages may be executed out of order from that shown ordiscussed, including substantially concurrent or in reverse order,depending on the functionality involved. Further, more or fewer blocksand/or functions may be used with any of the ladder diagrams, scenarios,and flow charts discussed herein, and these ladder diagrams, scenarios,and flow charts may be combined with one another, in part or in whole.

A block that represents a processing of information may correspond tocircuitry that can be configured to perform the specific logicalfunctions of a herein-described method or technique. Alternatively oradditionally, a block that represents a processing of information maycorrespond to a module, a segment, or a portion of program code(including related data). The program code may include one or moreinstructions executable by a processor for implementing specific logicalfunctions or actions in the method or technique. The program code and/orrelated data may be stored on any type of computer readable medium suchas a storage device including a disk or hard drive or other storagemedium.

The computer readable medium may also include non-transitory computerreadable media such as computer-readable media that stores data forshort periods of time like register memory, processor cache, and randomaccess memory (RAM). The computer readable media may also includenon-transitory computer readable media that stores program code and/ordata for longer periods of time, such as secondary or persistent longterm storage, like read only memory (ROM), optical or magnetic disks,compact-disc read only memory (CD-ROM), for example. The computerreadable media may also be any other volatile or non-volatile storagesystems. A computer readable medium may be considered a computerreadable storage medium, for example, or a tangible storage device.

Moreover, a block that represents one or more information transmissionsmay correspond to information transmissions between software and/orhardware modules in the same physical device. However, other informationtransmissions may be between software modules and/or hardware modules indifferent physical devices.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

The invention claimed is:
 1. A method comprising: determining, by ahead-mountable device (HMD), a measure of ambient light at each of oneor more ambient light sensors (ALSs) arranged on the HMD, wherein eachALS is configured to measure ambient light corresponding to a respectivelocation of one of a plurality of electromagnetic-radiationemitter/sensors (EESs) arranged on the HMD, and wherein ambient lightcomprises electromagnetic radiation other than electromagnetic radiationemitted by the plurality of EESs; based on the one or more determinedmeasures of ambient light at the one or more ALSs, determining, by theHMD, (i) a first number of EESs to use for emitting electromagneticradiation toward an eye and (ii) a second number of EESs to use fordetecting electromagnetic radiation reflected from the eye; selecting,by the HMD from among the plurality of EESs, a first group of particularEESs based at least on the first group having the first number of EESs;selecting, by the HMD from among the plurality of EESs, a second groupof particular EESs based at least on the second group having the secondnumber of EESs; causing, by the HMD, each of the particular EESs of thefirst group to emit electromagnetic radiation toward the eye; receiving,by the HMD from the particular EESs of the second group, data indicativeof electromagnetic radiation reflected from the eye; and determining, bythe HMD, a position of a portion of the eye based on the received data.2. The method of claim 1, further comprising: further based on the oneor more determined measures of ambient light at the one or more ALSs,determining by the HMD, for each of the particular EESs of the firstgroup, respective electromagnetic radiation characteristics to use whenemitting electromagnetic radiation toward the eye, wherein causing eachof the particular EESs of the first group to emit electromagneticradiation toward the eye comprises causing each of the particular EESsof the first group to emit toward the eye electromagnetic radiationhaving the determined respective electromagnetic radiationcharacteristics.
 3. The method of claim 2, wherein the respectiveelectromagnetic radiation characteristics comprise one or more of: (i) apower at which the electromagnetic radiation is emitted toward the eye,(ii) a duration for which the electromagnetic radiation is emittedtoward the eye, and (iii) a frequency at which the electromagneticradiation is emitted toward the eye.
 4. The method of claim 1, whereinthe portion of the eye comprises a pupil of the eye.
 5. The method ofclaim 1, wherein selecting the first group comprises for each particularEES from among the plurality of EESs: (i) if the determined measure ofambient light at a particular ALS associated with the particular EESexceeds a threshold measure of ambient light, including the particularEES in the first group, and (ii) if the determined measure of ambientlight at the particular ALS associated with the particular EES is belowthe threshold measure of ambient light, excluding the particular EESfrom the first group.
 6. The method of claim 1, wherein selecting thesecond group comprises for each particular EES from among the pluralityof EESs: (i) if the determined measure of ambient light at a particularALS associated with the particular EES exceeds a threshold measure ofambient light, including the particular EES in the second group, and(ii) if the determined measure of ambient light at the particular ALSassociated with the particular EES is below the threshold measure ofambient light, excluding the particular EES from the second group.
 7. Ahead-mountable device (HMD), comprising: a processor; a plurality ofelectromagnetic-radiation emitter/sensors (EESs); one or more ambientlight sensors (ALSs), wherein each ALS is configured to measure ambientlight corresponding to a respective location of one of the plurality ofEESs, and wherein ambient light comprises electromagnetic radiationother than electromagnetic radiation emitted by the plurality of EESs; anon-transitory computer-readable medium; and program instructions storedon the non-transitory computer-readable medium and executable by theprocessor to cause the HMD to perform functions comprising: determininga measure of ambient light at each of the one or more ALSs; based on theone or more determined measures of ambient light at the one or moreALSs, determining (i) a first number of EESs to use for emittingelectromagnetic radiation toward an eye and (ii) a second number of EESsto use for detecting electromagnetic radiation reflected from the eye;selecting, from among the plurality EESs, a first group of particularEESs based at least on the first group having the first number of EESs;selecting, from among the plurality of EESs, a second group ofparticular EESs based at least on the second group having the secondnumber of EESs; causing each of the particular EESs of the first groupto emit electromagnetic radiation toward the eye; receiving, from theparticular EESs of the second group, data indicative of electromagneticradiation reflected from the eye; and determining a position of aportion of the eye based on the received data.
 8. The HMD of claim 7,the functions further comprising: further based on the one or moredetermined measures of ambient light at the one or more ALSs, determinefor each of the particular EESs of the first group, respectiveelectromagnetic radiation characteristics to use when emittingelectromagnetic radiation toward the eye, wherein causing each of theparticular EESs of the first group to emit electromagnetic radiationtoward the eye comprises causing each of the particular EESs of thefirst group to emit toward the eye electromagnetic radiation having thedetermined respective electromagnetic radiation characteristics.
 9. TheHMD of claim 8, wherein the respective electromagnetic radiationcharacteristics comprise one or more of: (i) a power at which theelectromagnetic radiation is emitted toward the eye, (ii) a duration forwhich the electromagnetic radiation is emitted toward the eye, and (iii)a frequency at which the electromagnetic radiation is emitted toward theeye.
 10. The HMD of claim 7, wherein selecting the first group comprisesfor each particular EES from among the plurality of EESs: (i) if thedetermined measure of ambient light at a particular ALS associated withthe particular EES exceeds a threshold measure of ambient light,including the particular EES in the first group, and (ii) if thedetermined measure of ambient light at the particular ALS associatedwith the particular EES is below the threshold measure of ambient light,excluding the particular EES from the first group.
 11. The HMD of claim7, wherein selecting the second group comprises for each particular EESfrom among the plurality of EESs: (i) if the determined measure ofambient light at a particular ALS associated with the particular EESexceeds a threshold measure of ambient light, including the particularEES in the second group, and (ii) if the determined measure of ambientlight at the particular ALS associated with the particular EES is belowthe threshold measure of ambient light, excluding the particular EESfrom the second group.
 12. The HMD of claim 7, wherein the portion ofthe eye comprises a pupil of the eye.
 13. An article of manufactureincluding a non-transitory computer-readable medium having instructionsstored thereon that, if the instructions are executed by ahead-mountable device (HMD), cause the HMD to perform functionscomprising: determining a measure of ambient light at each of one ormore ambient light sensors (ALSs) arranged on the HMD, wherein each ALSis configured to measure ambient light corresponding to a respectivelocation of one of a plurality of electromagnetic-radiationemitter/sensors (EESs), and wherein ambient light compriseselectromagnetic radiation other than electromagnetic radiation emittedby the plurality of EESs; based on the one or more determined measuresof ambient light at the one or more ALSs, determining (i) a first numberof EESs to use for emitting electromagnetic radiation toward an eye and(ii) a second number of EESs to use for detecting electromagneticradiation reflected from the eye; selecting, from among the pluralityEESs, a first group of one or more particular EESs based at least on thefirst group having the first number of EESs; selecting, from among theplurality of EESs, a second group of particular EESs based at least onthe second group having the second number of EESs; causing each of theparticular EESs of the first group to emit electromagnetic radiationtoward the eye; receiving, from the particular EESs of the second group,data indicative of electromagnetic radiation reflected from the eye; anddetermining a position of a portion of the eye based on the receiveddata.
 14. The article of manufacture of claim 13, wherein the functionsfurther comprise: further based on the one or more determined measuresof ambient light at the one or more ALSs, determining, for each of theparticular EESs of the first group, respective electromagnetic radiationcharacteristics to use when emitting electromagnetic radiation towardthe eye, wherein causing each of the particular EESs of the first groupto emit electromagnetic radiation toward the eye comprises causing eachof the particular EESs of the first group to emit toward the eyeelectromagnetic radiation having the determined respectiveelectromagnetic radiation characteristics.
 15. The article ofmanufacture of claim 14, wherein the respective electromagneticradiation characteristics comprise one or more of: (i) a power at whichthe electromagnetic radiation is emitted toward the eye, (ii) a durationfor which the electromagnetic radiation is emitted toward the eye, and(iii) a frequency at which the electromagnetic radiation is emittedtoward the eye.
 16. The article of manufacture of claim 13, wherein theportion of the eye comprises a pupil of the eye.
 17. The article ofmanufacture of claim 13, wherein selecting the first group comprises foreach particular EES from among the plurality of EESs: (i) if thedetermined measure of ambient light at a particular ALS associated withthe particular EES exceeds a threshold measure of ambient light,including the particular EES in the first group, and (ii) if thedetermined measure of ambient light at the particular ALS associatedwith the particular EES is below the threshold measure of ambient light,excluding the particular EES from the first group.
 18. The article ofmanufacture of claim 13, wherein selecting the second group comprisesfor each particular EES from among the plurality of EESs: (i) if thedetermined measure of ambient light at a particular ALS associated withthe particular EES exceeds a threshold measure of ambient light,including the particular EES in the second group, and (ii) if thedetermined measure of ambient light at the particular ALS associatedwith the particular EES is below the threshold measure of ambient light,excluding the particular EES from the second group.